The present invention relates generally to a method for determining the porosity of an earth formation traversed by a borehole, and more particularly, to a method for measuring the total porosity of an earth formation.
Nuclear magnetic logging tools, such as disclosed in U.S. Pat. No. 4,933,638 issued to Kenyon et al., U.S. Pat. Nos. 5,055,787 and 5,055,788 issued to Kleinberg et al, measure the number and nuclear magnetic resonance (NMR) relaxation rates of hydrogen atoms in the pore space of rocks by measuring the amplitude and decay rate of signals resulting from pulse-echo sequences. The nuclear magnetic logging tools send a stream of RF-pulses into the formation and monitor the returning pulses which are called spin echoes. The signal measured by a nuclear magnetic logging tool, such as the CMR, mark of Schlumberger (Combined Magnetic Resonance) tool, formerly the PNMT, mark of Schlumberger (Pulsed Nuclear Magnetism Tool) is proportional to the mean density of hydrogen nuclei in the fluid that occupies the pore space. Since the hydrogen density in water and liquid hydrocarbons are approximately constant, the detected signal can be calibrated to give the volume fraction of the fluid occupying the pore space.
It has been shown that bound and unbound fluids can be distinguished by their relaxation times in water saturated rock samples. See C. Straley, C. E. Morriss, W. E. Kenyon, and J. J. Howard, NMR in Partially Saturated Rocks: Laboratory Insights on Free Fluid Index and Comparison with Borehole Logs, LOG ANALYST, January/February 1995, at 40 (paper presented at the 32nd Annual Logging Symposium, SWPLA, Jun. 16-19, 1991). Water that is bound to clay minerals, water in pores that are too small to be flushed by a feasible pressure gradient, and heavy (viscous) hydrocarbons all relax rapidly. Fluids that relax slowly have low viscosity and reside in large pores. Hence, the slowly relaxing fluids can be extracted from the formation, provided there is sufficient permeability.
The cutoff relaxation time, T.sub.c, distinguishing bound fluids from unbound fluids is empirically determined to be 50 msec for spin-lattice relaxation, T.sub.1, and 33 msec for spin-spin relaxation, T.sub.2, for water saturated sandstones and for 100 psi capillary pressure. With a T.sub.2 sensitivity limit of approximately 3 msec, in the presence of clays, silts, or microporosity, heretofore known NMR measurement techniques using the CMR tool may underestimate total rock porosity. Hydrogen nuclei in the rock matrix and some of the clay-bound water relax too rapidly and are not detected by the CMR tool. Thus, the CMR porosity measurement produces an effective porosity that does not include contributions from clay-bound water. See U.S. Pat. No. 5,291,137 issued to Robert Freedman; also see R. Freedman and C. E. Morriss, Processing of Data From an NMR Logging Tool, SPE 30560 (paper presented at the SPE Annual Technical Conference and Exhibition, Oct. 22-25, 1995). A need exists, therefore, for a total porosity measurement which is sensitive to fast relaxation times that can be associated with clay-bound water and includes, in addition to free fluid and capillary-bound porosity, the porosity of clay-bound water and microporosity.